A unique flat-file database module, written in pure perl. True multi-level hash/array support (unlike MLDBM, which is faked), hybrid OO / tie() interface, cross-platform FTPable files, ACID transactions, and is quite fast. Can handle millions of keys and unlimited levels without significant slow-down. Written from the ground-up in pure perl -- this is NOT a wrapper around a C-based DBM. Out-of-the-box compatibility with Unix, Mac OS X and Windows.

NOTE: 2.0000 introduces Unicode support in the File back end. This necessitates a change in the file format. The version 1.0003 format is still supported, though, so we have added a /db_version method. If you are using a database in the old format, you will have to upgrade it to get Unicode support.

NOTE: 1.0020 introduces different engines which are backed by different types of storage. There is the original storage (called 'File') and a database storage (called 'DBI'). q.v. "PLUGINS" for more information.

NOTE: 1.0000 has significant file format differences from prior versions. There is a backwards-compatibility layer at utils/upgrade_db.pl. Files created by 1.0000 or higher are NOT compatible with scripts using prior versions.

This is the traditional storage engine, storing the data to a custom file format. The parameters accepted are:

file

Filename of the DB file to link the handle to. You can pass a full absolute filesystem path, partial path, or a plain filename if the file is in the current working directory. This is a required parameter (though q.v. fh).

fh

If you want, you can pass in the fh instead of the file. This is most useful for doing something like:

my $db = DBM::Deep->new( { fh => \*DATA } );

You are responsible for making sure that the fh has been opened appropriately for your needs. If you open it read-only and attempt to write, an exception will be thrown. If you open it write-only or append-only, an exception will be thrown immediately as DBM::Deep needs to read from the fh.

file_offset

This is the offset within the file that the DBM::Deep db starts. Most of the time, you will not need to set this. However, it's there if you want it.

If you pass in fh and do not set this, it will be set appropriately.

locking

Specifies whether locking is to be enabled. DBM::Deep uses Perl's flock() function to lock the database in exclusive mode for writes, and shared mode for reads. Pass any true value to enable. This affects the base DB handle and any child hashes or arrays that use the same DB file. This is an optional parameter, and defaults to 1 (enabled). See "LOCKING" below for more.

When you open an existing database file, the version of the database format will stay the same. But if you are creating a new file, it will be in the latest format.

NOTE: This has only been tested with MySQL and SQLite (with disappointing results). I plan on extending this to work with PostgreSQL in the near future. Oracle, Sybase, and other engines will come later.

The recommended way to construct a DBM::Deep object is to use the new() method, which gets you a blessed and tied hash (or array) reference.

my $db = DBM::Deep->new( "foo.db" );

This opens a new database handle, mapped to the file "foo.db". If this file does not exist, it will automatically be created. DB files are opened in "r+" (read/write) mode, and the type of object returned is a hash, unless otherwise specified (see "OPTIONS" below).

You can pass a number of options to the constructor to specify things like locking, autoflush, etc. This is done by passing an inline hash (or hashref):

Notice that the filename is now specified inside the hash with the "file" parameter, as opposed to being the sole argument to the constructor. This is required if any options are specified. See "OPTIONS" below for the complete list.

You can also start with an array instead of a hash. For this, you must specify the type parameter:

Note: Specifying the type parameter only takes effect when beginning a new DB file. If you create a DBM::Deep object with an existing file, the type will be loaded from the file header, and an error will be thrown if the wrong type is passed in.

Alternately, you can create a DBM::Deep handle by using Perl's built-in tie() function. The object returned from tie() can be used to call methods, such as lock() and unlock(). (That object can be retrieved from the tied variable at any time using tied() - please see perltie for more info.)

There are a number of options that can be passed in when constructing your DBM::Deep objects. These apply to both the OO- and tie- based approaches.

type

This parameter specifies what type of object to create, a hash or array. Use one of these two constants:

DBM::Deep->TYPE_HASH

DBM::Deep->TYPE_ARRAY

This only takes effect when beginning a new file. This is an optional parameter, and defaults to DBM::Deep->TYPE_HASH.

autoflush

Specifies whether autoflush is to be enabled on the underlying filehandle. This obviously slows down write operations, but is required if you may have multiple processes accessing the same DB file (also consider enable locking). Pass any true value to enable. This is an optional parameter, and defaults to 1 (enabled).

The following parameters may be specified in the constructor the first time the datafile is created. However, they will be stored in the header of the file and cannot be overridden by subsequent openings of the file - the values will be set from the values stored in the datafile's header.

num_txns

This is the number of transactions that can be running at one time. The default is one - the HEAD. The minimum is one and the maximum is 255. The more transactions, the larger and quicker the datafile grows.

Simple access to a database, regardless of how many processes are doing it, already counts as one transaction (the HEAD). So, if you want, say, 5 processes to be able to call begin_work at the same time, num_txns must be at least 6.

This is the number of entries that can be added before a reindexing. The larger this number is made, the larger a file gets, but the better performance you will have. The default and minimum number this can be is 16. The maximum is 256, but more than 64 isn't recommended.

data_sector_size

This is the size in bytes of a given data sector. Data sectors will chain, so a value of any size can be stored. However, chaining is expensive in terms of time. Setting this value to something close to the expected common length of your scalars will improve your performance. If it is too small, your file will have a lot of chaining. If it is too large, your file will have a lot of dead space in it.

The default for this is 64 bytes. The minimum value is 32 and the maximum is 256 bytes.

Note: There are between 6 and 10 bytes taken up in each data sector for bookkeeping. (It's 4 + the number of bytes in your "pack_size".) This is included within the data_sector_size, thus the effective value is 6-10 bytes less than what you specified.

Another note: If your strings contain any characters beyond the byte range, they will be encoded as UTF-8 before being stored in the file. This will make all non-ASCII characters take up more than one byte each.

pack_size

This is the size of the file pointer used throughout the file. The valid values are:

small

This uses 2-byte offsets, allowing for a maximum file size of 65 KB.

medium (default)

This uses 4-byte offsets, allowing for a maximum file size of 4 GB.

large

This uses 8-byte offsets, allowing for a maximum file size of 16 XB (exabytes). This can only be enabled if your Perl is compiled for 64-bit.

This is a boolean option. When enabled, it allows external references to database entries to hold on to those entries, even when they are deleted.

To illustrate, if you retrieve a hash (or array) reference from the database,

$foo_hash = $db->{foo};

the hash reference is still tied to the database. So if you

delete $db->{foo};

$foo_hash will point to a location in the DB that is no longer valid (we call this a stale reference). So if you try to retrieve the data from $foo_hash,

for(keys %$foo_hash) {

you will get an error.

The external_refs option causes $foo_hash to 'hang on' to the DB entry, so it will not be deleted from the database if there is still a reference to it in a running program. It will be deleted, instead, when the $foo_hash variable no longer exists, or is overwritten.

This has the potential to cause database bloat if your program crashes, so it is not enabled by default. (See also the "export" method for an alternative workaround.)

With DBM::Deep you can access your databases using Perl's standard hash/array syntax. Because all DBM::Deep objects are tied to hashes or arrays, you can treat them as such (but see "external_refs", above, and "Stale References", below). DBM::Deep will intercept all reads/writes and direct them to the right place -- the DB file. This has nothing to do with the "TIE CONSTRUCTION" section above. This simply tells you how to use DBM::Deep using regular hashes and arrays, rather than calling functions like get() and put() (although those work too). It is entirely up to you how to want to access your databases.

You can even step through hash keys using the normal Perl keys() function:

foreach my $key (keys %$db) {
print "$key: " . $db->{$key} . "\n";
}

Remember that Perl's keys() function extracts every key from the hash and pushes them onto an array, all before the loop even begins. If you have an extremely large hash, this may exhaust Perl's memory. Instead, consider using Perl's each() function, which pulls keys/values one at a time, using very little memory:

while (my ($key, $value) = each %$db) {
print "$key: $value\n";
}

Please note that when using each(), you should always pass a direct hash reference, not a lookup. Meaning, you should never do this:

# NEVER DO THIS
while (my ($key, $value) = each %{$db->{foo}}) { # BAD

This causes an infinite loop, because for each iteration, Perl is calling FETCH() on the $db handle, resulting in a "new" hash for foo every time, so it effectively keeps returning the first key over and over again. Instead, assign a temporary variable to $db->{foo}, then pass that to each().

As with hashes, you can treat any DBM::Deep object like a normal Perl array reference. This includes inserting, removing and manipulating elements, and the push(), pop(), shift(), unshift() and splice() functions. The object must have first been created using type DBM::Deep->TYPE_ARRAY, or simply be a nested array reference inside a hash. Example:

In addition to the tie() interface, you can also use a standard OO interface to manipulate all aspects of DBM::Deep databases. Each type of object (hash or array) has its own methods, but both types share the following common methods: put(), get(), exists(), delete() and clear(). fetch() and store() are aliases to put() and get(), respectively.

new() / clone()

These are the constructor and copy-functions.

put() / store()

Stores a new hash key/value pair, or sets an array element value. Takes two arguments, the hash key or array index, and the new value. The value can be a scalar, hash ref or array ref. Returns true on success, false on failure.

$db->put("foo", "bar"); # for hashes
$db->put(1, "bar"); # for arrays

get() / fetch()

Fetches the value of a hash key or array element. Takes one argument: the hash key or array index. Returns a scalar, hash ref or array ref, depending on the data type stored.

Deletes one hash key/value pair or array element. Takes one argument: the hash key or array index. Returns the data that the element used to contain (just like Perl's delete function), which is undef if it did not exist. For arrays, the remaining elements located after the deleted element are NOT moved over. The deleted element is essentially just undefined, which is exactly how Perl's internal arrays work.

$db->delete("foo"); # for hashes
$db->delete(1); # for arrays

clear()

Deletes all hash keys or array elements. Takes no arguments. No return value.

This will compress the datafile so that it takes up as little space as possible. There is a freespace manager so that when space is freed up, it is used before extending the size of the datafile. But, that freespace just sits in the datafile unless optimize() is called.

optimize basically copies everything into a new database, so, if it is in version 1.0003 format, it will be upgraded.

import()

Unlike simple assignment, import() does not tie the right-hand side. Instead, a copy of your data is put into the DB. import() takes either an arrayref (if your DB is an array) or a hashref (if your DB is a hash). import() will die if anything else is passed in.

export()

This returns a complete copy of the data structure at the point you do the export. This copy is in RAM, not on disk like the DB is.

begin_work() / commit() / rollback()

These are the transactional functions. "TRANSACTIONS" for more information.

supports( $option )

This returns a boolean indicating whether this instance of DBM::Deep supports that feature. $option can be one of:

transactions

unicode

db_version()

This returns the version of the database format that the current database is in. This is specified as the earliest version of DBM::Deep that supports it.

For arrays, DBM::Deep supports all the common methods described above, and the following additional methods: length(), push(), pop(), shift(), unshift() and splice().

length()

Returns the number of elements in the array. Takes no arguments.

my $len = $db->length();

push()

Adds one or more elements onto the end of the array. Accepts scalars, hash refs or array refs. No return value.

$db->push("foo", "bar", {});

pop()

Fetches the last element in the array, and deletes it. Takes no arguments. Returns undef if array is empty. Returns the element value.

my $elem = $db->pop();

shift()

Fetches the first element in the array, deletes it, then shifts all the remaining elements over to take up the space. Returns the element value. This method is not recommended with large arrays -- see "LARGE ARRAYS" below for details.

my $elem = $db->shift();

unshift()

Inserts one or more elements onto the beginning of the array, shifting all existing elements over to make room. Accepts scalars, hash refs or array refs. No return value. This method is not recommended with large arrays -- see <LARGE ARRAYS> below for details.

$db->unshift("foo", "bar", {});

splice()

Performs exactly like Perl's built-in function of the same name. See "perldoc -f splice" for usage -- it is too complicated to document here. This method is not recommended with large arrays -- see "LARGE ARRAYS" below for details.

This causes DBM::Deep to flock() the underlying filehandle with exclusive mode for writes, and shared mode for reads. This is required if you have multiple processes accessing the same database file, to avoid file corruption. Please note that flock() does NOT work for files over NFS. See "DB OVER NFS" below for more.

You can explicitly lock a database, so it remains locked for multiple actions. This is done by calling the lock_exclusive() method (for when you want to write) or the lock_shared() method (for when you want to read). This is particularly useful for things like counters, where the current value needs to be fetched, then incremented, then stored again.

Due to Win32 actually enforcing the read-only status of a shared lock, all locks on Win32 and cygwin are exclusive. This is because of how autovivification currently works. Hopefully, this will go away in a future release.

Say you have an existing hash with nested hashes/arrays inside it. Instead of walking the structure and adding keys/elements to the database as you go, simply pass a reference to the import() method. This recursively adds everything to an existing DBM::Deep object for you. Here is an example:

This recursively imports the entire $struct object into $db, including all nested hashes and arrays. If the DBM::Deep object contains existing data, keys are merged with the existing ones, replacing if they already exist. The import() method can be called on any database level (not just the base level), and works with both hash and array DB types.

Note: Make sure your existing structure has no circular references in it. These will cause an infinite loop when importing. There are plans to fix this in a later release.

Calling the export() method on an existing DBM::Deep object will return a reference to a new in-memory copy of the database. The export is done recursively, so all nested hashes/arrays are all exported to standard Perl objects. Here is an example:

This makes a complete copy of the database in memory, and returns a reference to it. The export() method can be called on any database level (not just the base level), and works with both hash and array DB types. Be careful of large databases -- you can store a lot more data in a DBM::Deep object than an in-memory Perl structure.

Note: Make sure your database has no circular references in it. These will cause an infinite loop when exporting. There are plans to fix this in a later release.

DBM::Deep has a number of hooks where you can specify your own Perl function to perform filtering on incoming or outgoing data. This is a perfect way to extend the engine, and implement things like real-time compression or encryption. Filtering applies to the base DB level, and all child hashes / arrays. Filter hooks can be specified when your DBM::Deep object is first constructed, or by calling the set_filter() method at any time. There are four available filter hooks.

Your filter function will be called only when dealing with SCALAR keys or values. When nested hashes and arrays are being stored/fetched, filtering is bypassed. Filters are called as static functions, passed a single SCALAR argument, and expected to return a single SCALAR value. If you want to remove a filter, set the function reference to undef:

If you have a 64-bit system, and your Perl is compiled with both LARGEFILE and 64-bit support, you may be able to create databases larger than 4 GB. DBM::Deep by default uses 32-bit file offset tags, but these can be changed by specifying the 'pack_size' parameter when constructing the file.

DBM::Deep->new(
file => $filename,
pack_size => 'large',
);

This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words instead of 32-bit longs. After setting these values your DB files have a theoretical maximum size of 16 XB (exabytes).

You can also use pack_size => 'small' in order to use 16-bit file offsets.

Note: Changing these values will NOT work for existing database files. Only change this for new files. Once the value has been set, it is stored in the file's header and cannot be changed for the life of the file. These parameters are per-file, meaning you can access 32-bit and 64-bit files, as you choose.

Note: We have not personally tested files larger than 4 GB -- all our systems have only a 32-bit Perl. However, we have received user reports that this does indeed work.

If you require low-level access to the underlying filehandle that DBM::Deep uses, you can call the _fh() method, which returns the handle:

my $fh = $db->_fh();

This method can be called on the root level of the datbase, or any child hashes or arrays. All levels share a root structure, which contains things like the filehandle, a reference counter, and all the options specified when you created the object. You can get access to this file object by calling the _storage() method.

my $file_obj = $db->_storage();

This is useful for changing options after the object has already been created, such as enabling/disabling locking. You can also store your own temporary user data in this structure (be wary of name collision), which is then accessible from any child hash or array.

DBM::Deep has full support for circular references. Meaning you can have a nested hash key or array element that points to a parent object. This relationship is stored in the DB file, and is preserved between sessions. Here is an example:

Note: Passing the object to a function that recursively walks the object tree (such as Data::Dumper or even the built-in optimize() or export() methods) will result in an infinite loop. This will be fixed in a future release by adding singleton support.

As of 1.0000, DBM::Deep hass ACID transactions. Every DBM::Deep object is completely transaction-ready - it is not an option you have to turn on. You do have to specify how many transactions may run simultaneously (q.v. "num_txns").

Three new methods have been added to support them. They are:

begin_work()

This starts a transaction.

commit()

This applies the changes done within the transaction to the mainline and ends the transaction.

rollback()

This discards the changes done within the transaction to the mainline and ends the transaction.

Transactions in DBM::Deep are done using a variant of the MVCC method, the same method used by the InnoDB MySQL engine.

Right now, you cannot run a transaction within a transaction. Removing this restriction is technically straightforward, but the combinatorial explosion of possible usecases hurts my head. If this is something you want to see immediately, please submit many testcases.

If a client is willing to assert upon opening the file that this process will be the only consumer of that datafile, then there are a number of caching possibilities that can be taken advantage of. This does, however, mean that DBM::Deep is more vulnerable to losing data due to unflushed changes. It also means a much larger in-memory footprint. As such, it's not clear exactly how this should be done. Suggestions are welcome.

The techniques used in DBM::Deep simply require a seekable contiguous datastore. This could just as easily be a large string as a file. By using substr, the STM capabilities of DBM::Deep could be used within a single-process. I have no idea how I'd specify this, though. Suggestions are welcome.

Currently, the only contention resolution mechanism is last-write-wins. This is the mechanism used by most RDBMSes and should be good enough for most uses. For advanced uses of STM, other contention mechanisms will be needed. If you have an idea of how you'd like to see contention resolution in DBM::Deep, please let me know.

This section describes all the known issues with DBM::Deep. These are issues that are either intractable or depend on some feature within Perl working exactly right. It you have found something that is not listed below, please send an e-mail to bug-DBM-Deep@rt.cpan.org. Likewise, if you think you know of a way around one of these issues, please let me know.

The problem is one of synchronization. When the variable being referred to changes value, the reference isn't notified, which is kind of the point of references. This means that the new value won't be stored in the datafile for other processes to read. There is no TIEREF.

It is theoretically possible to store references to values already within a DBM::Deep object because everything already is synchronized, but the change to the internals would be quite large. Specifically, DBM::Deep would have to tie every single value that is stored. This would bloat the RAM footprint of DBM::Deep at least twofold (if not more) and be a significant performance drain, all to support a feature that has never been requested.

CODE

Data::Dump::Streamer provides a mechanism for serializing coderefs, including saving off all closure state. This would allow for DBM::Deep to store the code for a subroutine. Then, whenever the subroutine is read, the code could be eval()'ed into being. However, just as for SCALAR and REF, that closure state may change without notifying the DBM::Deep object storing the reference. Again, this would generally be considered a feature.

If you do my $x = $db->{foo};, then start a transaction, $x will be referencing the database from outside the transaction. A fix for this (and other issues with how external references into the database) is being looked into. This is the skipped set of tests in t/39_singletons.t and a related issue is the focus of t/37_delete_edge_cases.t

The current level of error handling in DBM::Deep is minimal. Files are checked for a 32-bit signature when opened, but any other form of corruption in the datafile can cause segmentation faults. DBM::Deep may try to seek() past the end of a file, or get stuck in an infinite loop depending on the level and type of corruption. File write operations are not checked for failure (for speed), so if you happen to run out of disk space, DBM::Deep will probably fail in a bad way. These things will be addressed in a later version of DBM::Deep.

Beware of using DBM::Deep files over NFS. DBM::Deep uses flock(), which works well on local filesystems, but will NOT protect you from file corruption over NFS. I've heard about setting up your NFS server with a locking daemon, then using lockf() to lock your files, but your mileage may vary there as well. From what I understand, there is no real way to do it. However, if you need access to the underlying filehandle in DBM::Deep for using some other kind of locking scheme like lockf(), see the "LOW-LEVEL ACCESS" section above.

Beware of copying tied objects in Perl. Very strange things can happen. Instead, use DBM::Deep's clone() method which safely copies the object and returns a new, blessed and tied hash or array to the same level in the DB.

my $copy = $db->clone();

Note: Since clone() here is cloning the object, not the database location, any modifications to either $db or $copy will be visible to both.

If you take a reference to an array or hash from the database, it is tied to the database itself. This means that if the datum in question is subsequently deleted from the database, the reference to it will point to an invalid location and unpredictable things will happen if you try to use it.

So a seemingly innocuous piece of code like this:

my %hash = %{ $db->{some_hash} };

can fail if another process deletes or clobbers $db->{some_hash} while the data are being extracted, since %{ ... } is not atomic. (This actually happened.) The solution is to lock the database before reading the data:

$db->lock_exclusive;
my %hash = %{ $db->{some_hash} };
$db->unlock;

As of version 1.0024, if you assign a stale reference to a location in the database, DBM::Deep will warn, if you have uninitialized warnings enabled, and treat the stale reference as undef. An attempt to use a stale reference as an array or hash reference will cause an error.

Beware of using shift(), unshift() or splice() with large arrays. These functions cause every element in the array to move, which can be murder on DBM::Deep, as every element has to be fetched from disk, then stored again in a different location. This will be addressed in a future version.

This has been somewhat addressed so that the cost is constant, regardless of what is stored at those locations. So, small arrays with huge data structures in them are faster. But, large arrays are still large.

If you pass in a filehandle to new(), you may have opened it in either a readonly or writeonly mode. STORE will verify that the filehandle is writable. However, there doesn't seem to be a good way to determine if a filehandle is readable. And, if the filehandle isn't readable, it's not clear what will happen. So, don't do that.

The problem is that the moment a reference used as the rvalue to a DBM::Deep object's lvalue, it becomes tied itself. This is so that future changes to $x can be tracked within the DBM::Deep file and is considered to be a feature. By the time the rollback occurs, there is no knowledge that there had been an $x or what memory location to assign an export() to.

NOTE: This does not affect importing because imports do a walk over the reference to be imported in order to explicitly leave it untied.

Stonehenge Consulting (http://www.stonehenge.com/) sponsored the developement of transactions and freespace management, leading to the 1.0000 release. A great debt of gratitude goes out to them for their continuing leadership in and support of the Perl community.